In an internal combustion engine, in particular in a multi-cylinder engine for a vehicle, a plurality of inlet pipes in the number corresponding to the number of the cylinders are provided for supplying air to a cylinder, and the internal combustion engine includes a surge tank connecting to the plurality of gathered inlet pipes. In addition, an electronically controlled fuel injection unit drives an auxiliary fuel injection valve called a cold start injector at the time of cold start, so as to attain excellent starting property (ignition quality, exhaust emission, or the like). The cold start injector is generally provided in the surge tank, and controlled in synchronization with a start switch based on detection of a temperature of engine cooling water. Normally, a single cold start injector is provided. Here, for uniform supply of a starter auxiliary fuel to all cylinders with a single cold start injector, whether or not an attachment position or a fuel injection direction of the cold start injector is appropriate is a factor significantly affecting the starting property.
Normally, one cold start injector having one injection hole is provided in the vicinity of a central portion of the surge tank, so as to inject the auxiliary fuel in a direction attaining uniform distribution of the fuel to all cylinders. In an inlet pipe having an asymmetrical shape, however, distribution of the auxiliary fuel tends to be unbalanced with only one cold start injector. Specifically, the auxiliary fuel is distributed more in a direction of travel of injection or in a direction of flow of intake air. In addition, in an engine having a surge tank including a plurality of chambers, it is difficult to uniformly supply the starting fuel to all cylinders without delay using one cold start injector. Publications shown below disclose related arts with regard to such a cold start injector.
Japanese Patent Laying-Open No. 58-107831 (document 1) discloses an appropriate position of a cold start injector in an internal combustion engine, in which when a load to the internal combustion engine is small, supply of a fuel and intake air to operating cylinders is cut off and exhaust is returned to non-operating cylinders, so as to perform a partial cylinder operation. A partial operation control type internal combustion engine disclosed in document 1 cuts off fuel supply and intake air into the operating cylinders when a light load is applied to the engine using an intake shut-off valve provided in an inlet pipe path through which an operation-side surge tank connecting to gathered inlet pipes of operating cylinders connects with an inlet-side surge tank connecting to gathered inlet pipes of the non-operating cylinders and returns the exhaust to the non-operating cylinders, so as to perform a partial cylinder operation. A cold start injector is disposed in the inlet pipe path, and a starter auxiliary fuel injected from the cold start injector impinges perpendicularly on a valve wall surface of the intake shut-off valve.
With the partial operation control type internal combustion engine, the starter auxiliary fuel injected from the cold start injector impinges perpendicularly on the valve wall surface of the intake shut-off valve, and the auxiliary fuel is reliably atomized and diffused so that it is uniformly distributed to the operating cylinders and the non-operating cylinders, thereby reducing a starting time. Here, oil mist or deposit adhered on a valve element surface is blown off, so that the valve element surface and an area around a valve shaft are constantly maintained clean, thereby preventing lowering of a shut-off function of the intake shut-off valve.
Japanese Patent Laying-Open No. 11-294225 (document 2) discloses a fuel injection unit for an internal combustion engine attaining a function for excellent distribution of a fuel using an auxiliary fuel injection valve shared by cylinders at the time of cold start and under high load. In the fuel injection unit for the internal combustion engine, an air intake path connecting to each cylinder is branched, and a collector extends in a direction of a cylinder row, through which one end, intake air is introduced. The fuel injection unit includes fuel supply means for injecting a fuel toward downstream with respect to an intake air current at least at the time of start and for injecting the fuel toward upstream with respect to the intake air current at least during a prescribed high-load operation, the fuel supply means being arranged on an intake air introduction side of the collector.
With the fuel injection unit for the internal combustion engine, at the time of start, in particular immediately after cranking, each cylinder takes in air that has originally been present in the collector. Accordingly, the fuel is injected from the upstream toward the downstream of the collector. That is, the entire air that has originally been present in the collector is mixed with fuel spray, whereby excellent starting property is attained. On the other hand, in the high-load operation, the fuel is injected toward the upstream so as to oppose to the intake air current. In this manner, the fuel can be distributed over a wide range on a cross-section of the intake air current, and mixing of the intake air current with the fuel spray is promoted, thereby attaining excellent distribution of the fuel to each cylinder.
On the other hand, in the partial operation control type internal combustion engine disclosed in document 1 described above, the auxiliary fuel injected from the cold start injector is carried away by the intake air flow, resulting in high concentration of an atmosphere in the downstream of the shut-off valve. That is, it is difficult to uniformly supply the auxiliary fuel to all cylinders.
In the fuel injection unit for the internal combustion engine disclosed in document 2, at the time of cold start, the fuel is injected from the cold start injector toward the downstream (cylinder side) of the intake air current. With such injection, however, it takes time for the auxiliary fuel to reach each cylinder, and starting with excellent response cannot be realized. In addition, as the inlet pipe has an asymmetrical shape, it is difficult to uniformly distribute the auxiliary fuel to each cylinder.
An engine having six cylinders or more sometimes adopts a variable induction system called ACIS (Acoustic Control Induction System). Here, pressure fluctuation occurs in the inlet pipe due to indirect intake strokes. The pressure fluctuation that still remains in the inlet pipe even after an intake valve is closed causes a pulsing effect, which in turn affects a next intake stroke. If the pressure fluctuation that remains after the intake valve is closed is in synchronization with the next intake stroke, a pressure at the time of opening of the valve is raised and an amount of intake air is increased, thereby improving a torque. In order to positively utilize this pulsation effect, the variable induction system switches an effective length of the inlet pipe path in accordance with a cycle of a pulsating flow that varies in accordance with an engine speed, so as to improve the torque at every engine speed.
Such a variable induction system is implemented, for example, by providing a partition wall in the surge tank, providing a valve of a butterfly type on the partition wall, and virtually varying an interval between cylinders by opening/closing the valve. In other words, two virtual lengths of an intake manifold are switched in order to improve intake efficiency in an entire range from low speed to high speed, thereby improving the torque.
The surge tank adopting such a variable induction system is constituted of a plurality of chambers. With the surge tank of such a shape, it is particularly difficult to uniformly supply the starter fuel to all cylinders without delay with a single cold start injector. Since a solution by increasing the number of the cold start injectors causes cost increase, adoption thereof is less likely.